Pernell Roberts, 81, a strapping actor who was an original cast member of the long-running TV western "Bonanza" and the star of the medical drama "Trapper John, M.D.," died of cancer Jan. 24 at his home in Malibu, Calif.

Mr. Roberts was a Broadway and film veteran before joining the cast of "Bonanza" in 1959. Until abruptly quitting the NBC series in 1965, he played the thoughtful, sensitive Adam Cartwright, one of three sons of a clan whose deeds on and around the Ponderosa ranch delighted audiences and made the series one of the most watched TV shows of the era.

Mr. Roberts rode the range with his two on-screen brothers -- Michael Landon as the brash Little Joe and Dan Blocker as the burly, gentle Hoss -- before hanging up his spurs at the end of the sixth season.

Mr. Roberts had lost his enthusiasm for the role just when the show, which aired until 1973, was at its peak. Fans enjoyed the family dynamics, the obvious bonding and the way the grown sons respected the authority of their father, Ben Cartwright, played by Lorne Greene.

For Mr. Roberts, it seemed too remote from reality. "Isn't it just a bit silly for three adult males to get father's permission for everything they do?" Mr. Roberts told The Washington Post in 1963. "I haven't grown at all since the series began four years ago. I have an impotent role. Everywhere I turn, there's the father image."

Finally, turning his back on Hollywood wisdom and well-meant advice, Mr. Roberts left the family's mythical domain, and his role was written out of the show. A Los Angeles Times critic said the move led most of Mr. Roberts's acting contemporaries to view him as "some kind of nut."

But Mr. Roberts was a thoughtful man. His first wife, Vera Mowry, was a college professor. He performed in "Othello" and "Antigone" in college, marched for civil rights in Selma, Ala., and was perhaps more in tune than many of his fellows with the spirit of the 1960s. "I left for my own good," he said.

Pernell Elvin Roberts Jr. was born in Waycross, Ga., on May 18, 1928, and attended Georgia Tech and the University of Maryland, where academics took a distant second to his burgeoning interest in theater.

In 1986, Rudy Pugliese, who had directed the theater group at College Park for years, said Mr. Roberts already "had it" while on campus. "He had that magnificent voice, a great build and chutzpah," Pugliese said. "Nothing could stop him. No one could put him down."

Mr. Roberts played tuba in the Marine Corps band at Quantico before focusing on a theater career. After appearing in 18 productions during two years at the newly formed Arena Stage in Washington, he headed to New York, where his credits included Mephistopheles in "Dr. Faustus" and a 1956 Drama Desk Award for his off-Broadway work in the title role of Shakespeare's "Macbeth."

He had roles on Broadway, too, before leaving in 1957 for Hollywood, where he appeared in films including "Desire Under the Elms," a 1958 adaptation of the Eugene O'Neill drama starring Sophia Loren, Anthony Perkins and Burl Ives.

Mr. Roberts's marriages to Mowry, Judith Roberts and Kara Knack ended in divorce. A son from his first marriage, Chris, died in 1989. Survivors include his fourth wife, Eleanor Criswell.

After abandoning the Stetson in "Bonanza," Mr. Roberts made dozens of film and TV appearances. His most prominent role was Dr. John McIntyre, the chief of surgery at a San Francisco hospital who confronted questions of life and death with a calm gravity on "Trapper John, M.D." The show, which was based on the character from the popular film and TV comedy "M*A*S*H," aired on CBS from 1979 to 1986 and brought Mr. Roberts an Emmy nomination for outstanding lead actor in a dramatic series.

Mr. Roberts had a robust baritone that he displayed on folk and western albums with cast members from "Bonanza." He also recorded an album of folk music, "Come All Ye Fair and Tender Ladies" (1963).